Polymeric Heart Valve Leaflets Tested in an Accelerated Wear Tester Revealed a Stable Craze Microstructure

Rheumatic and calcified aortic heart valve disease is a global health concern, impacting millions of individuals across various age groups. The gold standard medical treatments recommend replacing the sick heart valves with bioprosthetic valves that are chemically fixed using glutaraldehyde, commonly sourced from bovines or porcines. Clinical investigations over more than two decades have revealed that fixed tissues are prone to premature calcification and tearing, thereby limiting their durability. An innovative alternative approach involves biomolecule-enhanced polyethylene-based linear low-density polyethylene. LLDPE thin films have high tear strength and excellent flexibility, making them an appealing choice for developing heart valves. Nonetheless, during durability testing according to the ISO 5840-2005 standards, these leaflets exhibited premature failure. The leaflets consistently wear and tear around highly stressed commissure posts. Nine of these worn leaflets were retrieved from the failed valves and chemically etched. The semicrystalline LLDPE polymer underwent chemical etching using a standard 2% w/v permanganate etching solution, followed by multistep washing. SEM analysis of virgin LLDPE unveiled distinctive spherulitic structures consisting of well-organized lamellae with diameters of approximately 3 μm and dimensions below 100 nm. The etching process effectively eliminated low-energy amorphous regions, revealing the spherulites. A similar study was carried out on the damaged leaflets. The SEM images displayed signs of surface wear and aligned areas of polymeric material oriented perpendicular to the principal stress direction. Following etching, some of the built-up remained partially intact, while other areas exposed the crystals beneath them. Remarkably, one of the worn samples unveiled the Kramer craze microstructure "cross-tie" composed of aligned fibrils and interlinked fibrils. The spacing between cross-tie lamella ranges between 100 and 200 nm and the thickness remains at 40-80 nm. To the best of the author's knowledge, a cross-tie structure has only been theorized with indirect evidence collected from laboratory-grown crazes. These findings are further confirmed with SAXS.